Magenta -
I guess I read your post as speaking generally on the issue rather than more specifically. In return I responded more generally, not necessarily at you only but at people in general who believe I don't exist. (I am the OP of this topic by the way.)
I frequent this board every so often, sticking around for a few days, then I leave for a few months or so. I noticed you were a new user I hadn't recognized in the past (I see a lot of the same faces on here it seems).
Anyways, I agree there are people who claim to be ex-Christians who were never really saved to begin with. When I was a Christian I loved debating, and I ran across this guy who claimed to be an ex-Christian (ironically, at the time, I believed there was no such thing as true ex-Christians). He came from a Catholic family, although they never went to church except for major holidays. He never really prayed or thought about God in his day to day life. Yet he considered himself ex-Christian because he was born into a Catholic family but is now an atheist. Obviously, he was never a true genuine Christian in the first place. So I agree that not every claim is true.
Some time later, I was debating another atheist who was attending a Christian university. I asked him why he was attending a Christian university if he's an atheist. He said he wasn't an atheist when he first enrolled. He explained he was the son of a Baptist pastor and began telling me his testimony, explaining when he accepted Christ and what his Christian walk was like. He used language that I only really heard other Christians use. This guy definitely seemed different from others claiming to be ex-Christians. After talking with him many times after that, I had to accept the undeniable truth staring me straight in the face, here is a person who seemingly truly believed in Christ, who believed they had a real relationship with God, who now no longer believes. I just never knew people like that actually existed.
As far as my deconversion, it was not just one thing but several different things that led to me losing my faith. Imagine our beliefs are like pillars. Simple beliefs are small pillars that can be broken easily. "There is an apple in my refrigerator" is a simple belief. All you have to do is open the refrigerator and see if there is an apple. If there is no apple, that pillar of belief falls pretty easily. However, complex, or mega-beliefs, like "I believe that Jesus is my personal Lord and Savior" are held up by many pillars (smaller beliefs), so that if one particular belief comes under attack, or even crumbles, the other beliefs still hold up the mega-belief. It's only when a significant amount of these smaller beliefs that form the mega-belief get countered or corrupted that the mega-belief begins to waver and potentially crumble. What makes up these smaller beliefs? Some examples are faith, personal relationship, logical arguments, other Christians, personal experiences, prayer, morality, creation, the Bible, etc. It's weird thinking of something spiritual like belief in such a psychological way, but I think it's the best way to explain how it was not just one thing but many things that led to my deconversion. Needless to say this post is long enough, and it would take an equally long post to explain in detail the reasons behind my deconversion, so I will stop short of giving specifics at the moment and leave you with this general understanding.
To answer your last question, I was raised Baptist, but around 13 or 14 we started attending nondenominational churches, but they pretty much mirror Baptists in the majority of their theology. I also went to a Christian middle school/high school.